1. Field of the Invention
The present invention generally relates to a differential absorption laser radar apparatus and, more particularly, to a gas detection apparatus for measuring an optical gas concentration by using absorption of light, thereby detecting a gas leakage of a city gas, a chemical plant or the like.
2. Description of the Related Art
A gas such as methane has an absorption band for light having a specific wavelength in accordance with rotation of molecules or vibrations between constituent atoms.
Detection of methane gas will be described below as an example. Methane has absorption bands at wavelengths of 1.33 .mu.m 1.67 .mu.m and 3.39 .mu.m. By utilizing these absorption bands, various types of gas detection apparatuses using a differential absorption laser radar method have been proposed.
For example, a reference K. Uehara, "Alternate Intensity Modulation of a Dual-Wavelength He-Ne Laser for Differential Absorption Measurements", Appl. Phys. B 38, PP. 37 to 40 (1985) is known. This reference discloses as following. "A simple method is demonstrated for internal intensity modulation of the 3.391 and 3.392 .mu.m emissions of a He-Ne laser with equal amplitudes and 180.degree. out of phase to each other. A modulation amplitude of 0.7 mW peak-to-peak at 1 kHz for the individual emissions has been obtained from a laser plasma tube 50 cm long while maintaining the total-intensity modulation as low as 0.25/.mu.W for a signal averaging time of 1 s. This light source can greatly implify the setup and improve the sensitivity of differential absorption measurements for the methane remote sensing."
Other known references are Published Unexamined Patent Application Ser. Nos. 61-222289 and 62-98235. According to techniques of gas detection apparatuses described in these references, an infrared He-Ne laser for outputting a laser beam having a wavelength of 3.3922 .mu.m which is absorbed by methane and a laser beam having a wavelength of 3.3912 .mu.m which is almost not absorbed by methane is used as a light source, laser beams having the above two wavelengths are alternately radiated in the air by an equal power, and direct light or reflected light of the radiated beams is received. If methane is present in the air, a difference is produced between reception light signal levels of the two wavelengths of 3.3922 .mu.m and 3.3912/.mu.m. That is, in the techniques described in these references, the presence of methane in the air (optical path) or its concentration are detected by measuring the difference.
According to techniques of a gas detection apparatus described in Published Unexamined Japanese Patent Application No. 62-290190, a semiconductor laser having an oscillation wavelength near 1.33 .mu.m is used as a light source and modulated by two different current values having a predetermined current value as a center to oscillate at two wavelengths near 1.33 .mu.m, thereby detecting methane as described above.
In the laser apparatus using the infrared He-Ne laser, however, a large number of parts such as a gas absorption cell and a mirror must be used. Therefore, an arrangement of the apparatus is complicated, and the apparatus is weak against mechanical vibrations. In addition, since a bulky driving system must be used, the apparatus becomes large and expensive.
In the two-wavelength laser apparatus using the semiconductor laser, when a current for driving a laser is changed to change the wavelength, an output value of a laser beam is simultaneously changed. Therefore, the apparatus cannot be directly used as a light source for a differential absorption laser radar method. For this reason, in order to substantially equalize the output values of the two wavelength components, a complicated output adjusting means comprising a gas absorption cell, a mirror and a photosensor is required. Therefore, in consideration of deterioration over time and the like, it is difficult to manufacture a gas detection apparatus with high precision and reliability.